Semiconductor devices such as image sensors are widely used in digital still cameras, cellular phones, security cameras, as well as in, medical, automobile, and other applications. Complementary metal-oxide-semiconductor (“CMOS”) technology is used to manufacture lower cost semiconductor devices such as image sensors on silicon substrates. In a large number of image sensors, the image sensor commonly includes hundreds, thousand or even millions of light sensor cells or pixels. A typical individual pixel includes a micro-lens, a filter, a photosensitive element, a floating diffusion region, and one or more transistors for reading out a signal from the photosensitive element. As semiconductor devices such as image sensors become smaller and smaller, isolating electrical signals between electrical components within the semiconductor device becomes more important.
Fabricating semiconductor devices often includes forming trenches in a semiconductor layer of the semiconductor device. The trenches formed are used in a variety of applications including shallow trench isolation (“STI”). STI can help isolate adjacent electrical components disposed in the same semiconductor layer by reducing the electrical current leakage between components. In some applications, it is desirable to have many trenches in the same semiconductor layer, and for the trenches to have varying depths. For example, in an image sensor, it may be desirable for the trenches between each pixel in a pixel array to have a shorter depth than the trenches between the pixel array and a peripheral circuit region that contains circuit elements that control the pixel array. Therefore, with the demand for varying depth trenches in semiconductor devices, an inexpensive way to form precision trenches is desirable.